1. Field of the Invention
The present invention relates to a method and apparatus for controlling the air-fuel ratio in an internal combustion engine in which a fuel cut-off control is carried out.
2. Description of the Related Art
In general, fuel cut-off control is effected to stop the injection of fuel during deceleration, thereby improving fuel consumption. The control of the fuel cut-off depends upon the opening of a throttle valve, the engine speed, and the like. For example, when the throttle valve is completely closed and the engine speed is higher than a predetermined fuel cut-off engine speed, fuel cut-off is activated. Contrary to this, when the throttle valve is not completely closed or when the engine speed is lower than a predetermined fuel cut-off recovery engine speed, fuel cut-off is released. In this case, the fuel cut-off engine speed is higher than the fuel cut-off recovery engine speed, thereby obtaining the hysteresis characteristics of the engine speed. In addition, both the fuel cut-off engine speed and the fuel cut-off recovery engine speed are dependent upon engine state parameters such as the coolant temperature of the engine.
If such a fuel cut-off control is applied to a single point injection (SPI) type engine in which a combustion chamber of each cylinder is distant from an injection valve common for all cylinders, fuel adhered to the inner wall of an intake air manifold is also injected into the combustion chambers during a fuel cut-off period. As a result, the intake air manifold is completely drained, that is, no fuel adheres to the inner wall of the intake air manifold. Subsequently, even when the engine speed becomes lower than the fuel cut-off recovery speed, thereby restarting fuel injection, a long time period elapses before fuel saturation within the intake air manifold is achieved. That is, during such a long period, only a small amount of fuel is injected into the combustion chambers. Therefore, even if the engine is set in an acceleration mode during such a long period, the fuel within the combustion chambers is drained to increase the air-fuel ratio, thus deteriorating the drivability, i.e., generating lag or vibration in the engine.
To improve the drivability, there is known an internal combustion engine wherein fuel enrichment is carried out when the engine is switched from a deceleration mode to an acceleration mode after the fuel cut-off continues for a period longer than a predetermined period (see: Japanese Examined Patent Publication No. 56-42739).
In the above-mentioned prior art, however, a predetermined amount of fuel enrichment is injected into the engine irrespective of the duration of the period after the fuel cut-off recovery, and an excess of fuel may be injected into the engine, which is disadvantageous from the viewpoint of fuel consumption.